Lenses usable in step and repeat microlithography must have high numerical apertures and must have fields as flat as possible in order to reproduce the reticle pattern accurately onto the surface of a wafer.
In conventional optical designs for such applications, the third order Petzval curvature is left partly under-corrected. That is, it is curved inward toward the lens so that at some point in the field the third order Petzval just balances the fifth order Petzval. The fifth order Petzval is usually positive, or over corrected, and curves away from the lens. Thus, at the point where the third and fifth order Petzval just balance, the curvature of field is zero and the position of best focus corresponds to the paraxial focus. By choosing a different focus on axis than the best focus, a better balance can be achieved so that, at two points in the field, the position of best focus corresponds to the image plane.
The position of best focus includes factors other than Petzval curvature. However, Petzval curvature has a strong influence and results in a field which, at low values of image height curves inward but, at large values of image height, curves outward from the lens. It has been discovered that a single strong field lens may be used to correct the third order Petzval curvature. However, it has also been determined that no amount of bending of the strong field lens will correct the fifth order Petzval and, in fact, only adds additional over correction. An example of a field flattener employed to correct third order field curvature is disclosed in U.S. Pat. No. 2,846,923 of A. W. Tronnier. In the system disclosed in this patent, third and fifth order distortion remained when such a field flattener was employed.